BRACHIOPODA. 303 



liver ; the intestine of Lingiila is reflected dorsally, slightly 

 convoluted, and terminates between the mantle-lobes on the right 

 side. In Orbicnla it is reflected ventrally, and passes straight 

 to the right, ending as in Lingula. In Terebratula, Rhynchonella, 

 and probably all the articulated Brachiopoda, the intestine is 

 simple and reflected ventrally, passing through a notch or fora- 

 men in the hinge-plate, and ending behind the ventral insertion 

 of the adductor muscle. 



The circulatory system is far less complex than was formerly 

 supposed, and does not differ greatly from the same system in 

 the Tunicata. The heart is placed on the dorsal surface of the 

 stomach, and consists of a simple, unilocular, pyriform vesicle 

 without any auricle. From it the blood is propelled through 

 four channels to the organs of reproduction and to the mantle ; 

 and its flow is probably assisted by a number of subsidiary 

 pulsatile vesicles situated on the main arterial trunks. It then 

 courses through the plexus of lacunes in the pallial sinuses and 

 lobes ; turns back through the lacunes of the parietes into the 

 system of visceral lacunes. It probably enters the liver, and 

 ultimately fiinds its way back into the heart through the 

 branchio-sj^stemic vein. There is, however, another and more 

 important blood current, which traverses the whole length of 

 the brachial canal, and penetrates to the extremities of the cirri, 

 before it joins the current returning from the visceral lacunes 

 and flows with it into the branchio-S3'stemic vein. The blood 

 which has passed through the brachial canal is far more highly 

 oxygenated than the blood which has flowed through the pallial 

 membranes. There seems to be strong evidence that the so-called 

 arms, which serve to bring food to the creature's mouth b^' the 

 means before noticed, also subserve the purpose of respiratory 

 organs. The mantle is an accessory breathing-organ. It attains 

 its highest development as such in Lingula, but even in this 

 genus the brachial apparatus performs the chief part in oxygen- 

 ating the blood. 



There is another system of canals which take their rise from 

 the visceral cavity. What its function is has not been deter- 

 mined; it is not the blood system as was formerly imagined, 

 and has no connection with it. The perivisceral cavity and the 

 visceral lacunes which diverge from it may, it is thought, be 

 homologous to the water-vascular system in Polyzoa, the 

 function of which is probably to evacuate the etfete nitrogenized 

 products which have been eliminated from the blood. Conse- 

 quentlj^ it would perform the offices both of the kidney and the 

 renal organs. 



The generative organs occupy the great pallial sinuses, and 

 the sexes are separate. In the articulated brachiopods the 



